Radial bearing assemblies are utilized in many machine and tool applications to reduce rotational friction and to radially support loads. Such radial assemblies are often employed in downhole drilling motors used when drilling boreholes in the earth for oil and gas exploration. Some radial bearing assemblies employ a pair of opposing support members each having an opposing groove or bearing race. Bearing balls are positioned in the opposing races between the support members and as one support member rotates with respect to the other, the balls rotate in the races to reduce rolling resistance and friction between the opposing support members. Other radial bearing assemblies employ elongated rollers in place of the bearing balls.
Other radial hearing assemblies use bearing, inserts comprised of flat hard discs or plates to line the surface of opposing members. The hard smooth surface of the inserts reduce resistance and friction between the opposing support members as these support members rotate with respect to each other. Such bearing inserts are typically made of a hard carbide such as tungsten carbide, titanium carbide, silicon carbide, diamond silicon carbide composites, polycrystalline cubic boron nitride, or polycrystalline diamond, provide high strength, wear resistance and low friction between rotating support members.
When radial bearing assemblies are constructed with such baring inserts, each individual insert is placed in a bearing hole or aperture on the surface of the support members and attached by brazing or spot welding the insert in place on the support member or to a corresponding bearing ring positioned adjacent to the support member. A hearing hole or aperture is required to for attaching the disc to the bearing surface because of the difficulties encountered when welding or brazing a flat disc to a round bearing surface. Further, the machining of individual holes or apertures for the inserts and then spot welding or brazing each individual insert in place in its aperture is time consuming and expensive.
The geometry of a wellbore often places restrictions on the dimensions of downhole tools and equipment that may be used. The corresponding dimensional restrictions on radial bearings used in such downhole drilling tools and equipment impose quality control and machining difficulties that increase the cost of manufacturing these radial bearings. Applicants' U.S. Pat. No. 8,770,845 B1 disclosed a radial bearing assembly for downhole drilling equipment fabricated by inserting multiple flat disc shaped inserts into flat-bottomed retainer slots machined onto a tubular support and simultaneously brazing, these disc inserts within the slots in order to reduce the cost of bearing manufacture. While such bearings are now more efficiently manufactured, the cost of machining the flat-bottomed retainer slots is still time consuming and costly and often requires specialized machining equipment.
The rotary bearing assemblies used in downhole drilling motors typically utilize relatively thin and flat disc inserts. Thin and flat insert discs are susceptible to bending under the stresses induced during use of the drilling motors. Residual stresses induced during the manufacturing process including stress typically contained between the hard thin inserts and the steel body of the bearing assemblies can contribute to premature cracking of the disc inserts. Even excluding such manufacturer stress, the bearing surfaces of the disc inserts will wear down over time under the stresses of continual use. Such wear increases the likelihood of bending stress that can cause cracking on the bearing surfaces of the disc insert. Such cracking is a major problem as it will increase friction at the bearing surfaces that quickly lead to premature wear and eventual failure of the bearing assembly.
Further, disc inserts have sidewalls that extend perpendicular to its bearing surface. Brazing material around the sidewalls provides a mechanical grip on the disc inserts to permanently bond them in place. As the disc inserts wear during use, the sidewalls of the disc inserts become thinner and their gripping surface area at the brazing material is reduced. As these disc inserts wear under the stresses of use, risk of such disc inserts being dislodged from the brazing material and the bearing assembly is increased. The loss of one or more disc inserts from a radial bearing assembly will greatly increase the wear on the bearing surfaces of the remaining disc inserts and reduce the life of the bearing assembly. A lost disc insert may also migrate with the circulating wellbore fluid to another part of the downhole tool in which the bearing is employed, or to another wellbore tool, increasing the risk of tool malfunctions and resulting damage to the wellbore.
There is also an inherent limit on the number of discs that may be packed together in slots or grooves in a bearing assembly. It is advantage to tightly pack the bearing inserts into the slots or grooves in order to increase the bearing surface area provided by the discs. There is a need for a method of manufacturing radial bearing assemblies that will maximize the bearing surface area provided by the bearing inserts.
Consequently a need exists for a method that will reduce the cost of manufacturing radial bearing assemblies. There is also a need for a radial bearing assembly that will enhance the retention of the bearing inserts used in the bearing assembly under the stress encountered during use. There is a further need for a bearing assembly that will allow the bearing surface area of the bearing inserts to be maintained or enhanced as the bearing assembly is being used.